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Some Issues about Force and Newton’s Laws of Force.
1. State Newton’s Third law of motion in your own words and then as an expression. Explain any symbols you
use.
2. Whenever we wish to consider a situation which we want to address in terms of physics, we must “define” the
“system” we will be considering. Think about “defining the system” very carefully and answer the following
questions with explanation or justifications.
(a) Can you pull up a pencil?
(b) Can you pull up yourself by your boot straps?
(c) Earth pulls Moon toward Earth. Moon pulls Earth toward Moon. This is called “Gravitational Force” which
we will study in detail in later on. Which object pulls more than the other, given that the mass of Earth is
much greater than that of Moon?
(d) You have mass. Therefore, just like the previous question, Earth pulls you toward Earth (downward) and
you pull Earth toward you (upward). Which force is greater?
Vector Diagrams.
An important reason to have an “accurate” vector diagram in this class and in all physics classes for that
matter, is the diagram helps us to “see” something you could not “see” before. For example, try this
problem in your head to see if you can answer. If you can’t answer in a minute, go to #8.
There is a 10-kg block on an inclined plane. Also, there is 50 N of friction between the block and the
plane and the normal force on the block do to the incline is 70.7N. What is the acceleration of the
block?
It is very confusing if you try to do everything in your head. There is an easier way to do this problem. But before
that, you have to practice the following exercises. Be patient!
In each of the following, assume the object is within Earth’s gravitational field and that any motion will be
constrained by the displayed geometry.
8. Draw all the forces applied to (or acting on) the object in each case. These are very important practices for you
to be able to see applied forces. You must determine the forces based on the conditions given.
Case 2: v  0 m/sec, a = 0 m/sec2
Case 1: v = 0 m/sec, a = 0 m/sec2
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FORCE AND NEWTON’S LAWS
Case 3: With no friction
Case 4: With friction, a  0 m/sec2
Case 5: a = 0 m/sec2
Case 6: v = 0 m/sec, a = 0 m/sec2
Focus on the ball
Case 7: v = 0 m/sec, a = 0 m/sec2
Focus on the block
Case 8: Free fall
Case9: Terminal Speed
Case 10: Between Free fall and
Terminal speed
9. #8 was to practice determining and drawing “applied forces”. Now, we are going to practice adding the vectors
we have drawn to find the resultant force. Use the “parallelogram” method to be discussed in class.
Case 2: v  0 m/sec, a = 0 m/sec2
Case 1: v = 0 m/sec, a = 0 m/sec2
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FORCE AND NEWTON’S LAWS
Case 3: With no friction
Case 4: With friction, a  0 m/sec2
Case 5: a = 0 m/sec2
Case 6: v = 0 m/sec, a = 0 m/sec2
Focus on the ball
Case 7: v = 0 m/sec, a = 0 m/sec2
Focus on the block
Case 8: Free fall
Case9: Terminal Speed
Case 10: Between Free fall and
Terminal speed
After being able to draw all applied forces to an object, you can now answer the question that you could not answer
before.
10. There is a 10-kg block on an inclined plane. Also, there is 50 N of friction between the block and the plane and
the normal force on the block due to the plane is 70.7N. What is the acceleration of the block?
Before you attack the problem, write down the steps that you should take to solve this kind of problem.
Step1 –
Step 2 –
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FORCE AND NEWTON’S LAWS
Step 3 –
Step 4 –
Step 5 –
(add more steps as needed)
11. If you can do this one “from scratch” without looking at previous notes, you understand. If you have to look
back to your notes, you need more study.
A 10.0 kg block is on an inclined plane. If the friction between the block and the plane is 50 N, and the normal force
between the block and incline is 70.7N, what is the acceleration of the block?
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